Different historical objects are considered the same one if used with `as_of`

Probably this behavior is expected, but it is surprising and may should be documented

Let's say that I have an object that has been modified multiple times:

```
 my_object.as_of(1.month.ago) == my_object.as_of(1.minute.ago)
```

Returns `true`, because chronomodel does not check equality of `hid` when it is not an historical object
